Ugh, I hadn't read that. I'm running a "Fusion drive" in my '09 mini; everything set up correctly and APPEARS to be working as planned, but I haven't filled up my SSD yet (everything lives on my server). Gonna have to fill that SSD up and see what happens...
@OP, when I created mine it simply merged the volumes. My guess would be that YES, you could use the three drives, but your RAID0 would be gone; you'd simply have a 3 drive Fusion setup.